PC cards such as PCMCIA, 16-bit PC Card, Cradbus Card, ExpressCard, Compact Flash card and other PC/PDA add-on cards are manufactured in a variety of sizes and used as expansion boards for PCs, PDAs etc. The PC cards generate heat that has to be dissipated to avoid damage to the PC components or interruption of the application. Typically, such PC cards have a thermal dissipation capacity of about 1˜3 Watts (W). This thermal dissipation capacity is adequate for many applications for which the PC card is used. For example, a 2 W thermal dissipation capacity on a wireless PC card is satisfactory for surfing the web or transmission of e-mail, or other files of limited size. However, some applications may result in exceeding the thermal dissipation capacity of the PC card. For example, continued data transmission at high rates with maximum RF radiated power can exceed the thermal dissipation capacity of the PC card.
Additionally, the processing power requirements, and hence the heat generation, for wide area broadband wireless devices is increasing. For example, 3 G (an ITU specification for third generation wireless technology) may provide bandwidth of 2 Mbps or more. Since PCMCIA and other PC add-on card standards were not specifically defined for such applications, thermal dissipation is problematic when designing wireless modems for the mobile host devices, especially for smaller form factor PC cards such as ExpressCard/34.
Advances in semiconductor technology have lowered the power consumption per MIPS, but not enough to keep heat generation within acceptable levels, nor has the efficiency of transmitters used on wireless devices improved significantly. For example, according to the current PCMCIA PC Card standard, when a PC Card generates 3 W (i.e., thermal dissipation capacity) at the maximum host slot temperature of +65 C, the external card temperature will reach +90 C. In this case the card internal temperature would exceed +90 C. A typical class 12 GPRS (50% time burst transmit) wireless PC card with 5 W average DC power draw at 31.8 dBm RF power radiated would exceed +85 C internal temperature at only +60 C host slot temperature.
Compact Flash card technology shares the same interface standard as the 16-bit PC card, but with smaller size. When a typical class 10 GPRS wireless Compact Flash card is used in a PDA Compact Flash slot, similar thermal dissipation problems are observed.
Moreover, the size of next generation of PC cards (e.g., ExpressCard) is decreasing with the size of the mobile computing devices (e.g., laptop PCs, PDAs, etc.) in which they are used. The thermal dissipation capacity of the host slot on such devices is reduced accordingly and the thermal dissipation capacity of the card is reduced. For example, the ExpressCard has a thermal dissipation capacity of only 2.1 W.
Currently, excessive heat (exceeding the thermal dissipation capacity) is addressed through thermal management mechanisms. For example, the temperature of the card is monitored and if the temperature exceeds a specified threshold, the transmission power and/or transmission rate is reduced. Such thermal management mechanisms are disadvantageous in that they result in performance degradation on data throughput or radio cover range in balanced or upload oriented broadband wireless applications, such as FTP upload, video conference, wireless video surveillance, wireless digital camera, etc.